The present invention relates to a method for the emergency start of a combustion engine in the event of a failed speed sensor or speed sensors, to permit limited vehicle operation. If the speed sensor fails, no allocation of a camshaft signal of a phase sensor to the crankshaft position is possible during vehicle start.
German Published Patent Application No. 40 26 232 relates to a device for monitoring a speed sensor. The device includes a starter having a speed sensor that supplies an output signal as a function of the speed. A device for measuring the vehicle system voltages is provided, and a control unit is provided, in which the output signal of the speed sensor is set in relation to the vehicle system voltage, and a malfunction of the speed sensor may be detectable. The characteristic curve of the vehicle system voltage is evaluated during the starting process of the combustion engine for fault detection. A fault detection is triggered only when a characteristic curve of the vehicle system voltage that is typical for the starting process is detected and, at the same time, no output signal of the speed sensor is detected.
If a speed sensor, which continuously samples a trigger wheel at the crankshaft of a combustion engine, fails, vehicle operation, under emergency conditions, could also occur on the basis of an evaluation of phase sensor signals. A speed signal, with which a limited vehicle operation (limp home) may be possible, may be simulated from the phase sensor signal. If, alternatively, the combustion engine is operated with camshaft control, the location of the camshaft, that is, the position of the camshaft, may be unknown during vehicle start, since the phase sensor is mounted at the indefinitely positioned camshaft. Fluctuations may arise in an angular range up to 40xc2x0 arc of crankshaft rotation.
During the start of the vehicle, no allocation of the camshaft signal and of the phase sensor to the crankshaft position may be possible. Therefore, neither appropriate injection nor correspondingly matched ignition may be carried out by further systems of the fuel injection system. Consequently, a start of the combustion engine with camshaft control may be impossible if the speed sensor fails.
An object of an exemplary embodiment of the present invention is to ascertain the crankshaft position by evaluating the battery voltage during starter operation, in the event of a failed speed sensor.
Due to actuation of the starter, which is supplied electricity by a vehicle battery, cyclically repeating compression and decompression phases occur in the individual cylinders of the combustion engine, whether the vehicle is, for example, a 4-cylinder or 6-cylinder combustion engine. The end points of the compression and decompression phases, respectively, of the individual cylinders are determined at the positions of bottom dead center (BDC) and top dead center (TDC). According to the load of the starter during starter operation, which results from the compression and decompression phases of the individual cylinders, the battery current of an energy accumulator feeding the starter assumes a characteristic curve, from which the positions of the respective TDCs and BDCs may be determined. If the starter has finished a number of crankshaft revolutions, it may be possible to reliably allocate the individual TDCs and BDCs to ascertained maximums and minimums, respectively, of the battery current characteristic curve. If the allocation of maximums and minimums to the respective TDCs and BDCs is ensured, injection and ignition at corresponding cylinders may be effected by engine management, according to an injection and ignition sequence stipulated in engine control electronics.
The positions of TDC and BDC, respectively, ascertained from the maximums and minimums of the battery voltage characteristic, may be stored by a corresponding correlation table or a program map repeating the speed/load performance, and kept for future purposes.